1. Field of the Invention
The present invention relates to an optical waveguide device.
2. Related Art Statement
It is proposed a structure of lowering the thickness of an optical waveguide substrate to attain the velocity matching of microwave and light wave, so as to widen the modulation bandwidth of an optical modulator.
In such kind of a structure utilizing a thinner optical waveguide substrate, it is required to lower the thickness of the substrate around an optical waveguide to a value of, for example 10 μm, for satisfying the velocity matching condition. Such thin optical waveguide may result in the flattening of optical mode field pattern and transmission loss of light due to surface roughness and damage caused by the processing for producing the thin substrate and a groove. The assignee filed a Japanese Patent publication 2002-169133A disclosing two-step groove structure for preventing the flattening of optical mode field pattern and the transmission loss.
According to a device described in Japanese Patent publication H09-211402A, an air layer is provided in a reinforcing substrate to satisfy the velocity matching condition. Further, according to a device described in Japanese Patent publication 2004-341147A, a plate-shaped optical waveguide substrate having a thickness of 30 μm is bonded onto a reinforcing substrate to integrate them.
Further, Japanese Patent publication H04-355714A describes a traveling wave type optical modulator having a crossing part where two optical waveguide branches intersect each other. According to this type of optical modulator, the optical waveguide is folded at one edge of a substrate to lower the substrate length required for applying necessary intensity modulation.